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Molards as an indicator of permafrost degradation and processes

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Morino, Costanza; Conway, Susan J.; Sæmundsson, Þorsteinn; Kristinn Helgason, Jón; Hillier, John; Butcher, Frances E.G.; Balme, Matthew R.; Jordan, Colm and Argles, Tom (2019). Molards as an indicator of permafrost degradation and landslide processes. and Planetary Science Letters, 516 pp. 136–147.

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Earth and Planetary Science Letters

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Molards as an indicator of permafrost degradation and landslide processes ∗ Costanza Morino a,b, , Susan J. Conway b, Þorsteinn Sæmundsson c, Jón Kristinn Helgason d, John Hillier e, Frances E.G. Butcher f, Matthew R. Balme f, Colm Jordan g, Tom Argles a a School of Environment, Earth & Ecosystem Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK b Laboratoire de Planétologie et Géodynamique de Nantes UMR-CNRS 6112, 2 rue de la Houssinière, BP 92208, 44322 Nantes Cedex 3, France c Department of Geography and Tourism, University of Iceland, Askja, Sturlugata 7, IS-101 Reykjavík, Iceland d Icelandic Meteorological Office, Avalanche Centre, Suðurgata 12, Ísafjörður, Iceland e Geography and Environment, Loughborough University, Loughborough, LE11 3TU, UK f School of Physical Sciences, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK g British Geological Survey, Environmental Science Centre, Keyworth, Nottingham NG12 5GG, UK a r t i c l e i n f o a b s t r a c t

Article history: Molards have been defined in the past as conical mounds of debris that can form part of a landslide’s Received 15 February 2018 deposits. We present the first conclusive evidence that molards in permafrost are cones of loose Received in revised form 20 March 2019 debris that result from thawing of frozen blocks of ice-rich mobilised by a landslide, and hence Accepted 27 March 2019 propose a rigorous definition of this landform in permafrost environments. We show that molards can Available online xxxx be used as an indicator of permafrost degradation, and that their morphometry and spatial distribution Editor: J.P. Avouac give valuable insights into landslide dynamics in permafrost environments. We demonstrate that molards Keywords: are readily recognisable not only in the field, but also in remote sensing data; surveys of historic aerial molards imagery allow the recognition of relict molards, which can be used as an indicator of current and past permafrost permafrost conditions. The triggering of as a result of permafrost degradation will arguably landslide occur more often as global atmospheric temperatures increase, so molards should be added to our Iceland armoury for tracking change, as as helping us to understand landslide-related hazards. Finally, we have also identified candidate molards on Mars, so molards can inform about landscape evolution on Earth and other planetary bodies. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Jermyn and Geertsema, 2015), up to 12 m wide (Milana, 2016), have a single, central, rounded to pointed summit (Jermyn and Geertsema, 2015;Xu et al., 2012), and flank slope angles of Glacial and periglacial environments are particularly sensitive ◦ ◦ to the effects of (Haeberli and Beniston, 1998; 27 -45 (Cassie et al., 1988; McConnell and Brock, 1903). Molards Hinzman et al., 2005). However, few easily recognisable land- have been found in the distal zones, at the margins of the dis- forms in cold landscapes are reliable indicators of increasing placed mass, and/or below the main scarp of landslides (Cruden, atmospheric temperature. Here, we propose that molards, land- 1982; Geertsema et al., 2006b). forms poorly reported in the literature, can help to fill this Molards have recently been identified in a variety of periglacial gap. Molards have been defined as conical mounds occurring environments, including northern , the , and the in landslide deposits (Brideau et al., 2009; Cassie et al., 1988; peaks of south-east (Jermyn and Geertsema, 2015; Milana, Cruden, 1982; Geertsema et al., 2006b;Goguel and Pachoud, 1972; 2016;Xu et al., 2012). Three recent studies have hypothesised Jermyn and Geertsema, 2015;Lyle et al., 2004; McConnell and a link between molards and permafrost degradation (Brideau et Brock, 1903; Milana, 2016; Mollard and Janes, 1984;Xu et al., al., 2009;Lyle et al., 2004; Milana, 2016). It is proposed that 2012). They are generally ∼0.3-12 m high (Brideau et al., 2009; movement of landslide material in permafrost regimes causes blocks of frozen material to detach and be transported downs- lope (Brideau et al., 2009;Lyle et al., 2004). When the blocks come to rest, the ground ice cementing them thaws, leaving con- * Corresponding author at: Laboratoire de Planétologie et Géodynamique, Univer- sité de Nantes, Bâtiment 4, 2 Chemin de la Houssinière, 44300 Nantes, France. ical mounds of rocks and debris. Although no previous studies E-mail address: [email protected] (C. Morino). have observed the full cycle of molard evolution, original ground https://doi.org/10.1016/j.epsl.2019.03.040 0012-821X/© 2019 Elsevier B.V. All rights reserved. C. Morino et al. / Earth and Planetary Science Letters 516 (2019) 136–147 137

Fig. 1. The field sites. In the centre, an inset of the coastline of Iceland, with the location of the Móafellshyrna landslide as “a” and of the Árnesfjall landslide as “b”. a,The Móafellshyrna landslide photographed in summer 2015, with an inset showing the molards. b,The Árnesfjall landslide photographed in summer 2016, with an inset showing the molards. c, A molard in Árnesfjall photographed 2yrs after the landslide event and preserving the original preferential orientation of the source material (field notebook 19 × 13 cm as scale); note that the surface is clast-supported. d, A molard few days after the occurrence of Árnesfjall landside, preserving abundant fine deposits, person for scale. ice contents of 50-80% have been estimated (Brideau et al., 2009; ity in the release of the Móafellshyrna landslide (Morino, 2018; Milana, 2016)as sufficient for blocks of ice-cemented sediments to Sæmundsson et al., 2018). Both landslides could be simply clas- survive both initial failure and transport. sified as debris slides (Varnes, 1978), but we show that molards We studied molards found in two different landslides in Ice- allow the identification of additional failure dynamics. land: the first is located on the north-western side of Móafell- We combine field and remote sensing studies of molards to shyrna Mountain (Tröllaskagi peninsula, Fig. 1a), and the second constrain their complete evolution, providing the first observations on the north-facing side of the Árnesfjall Mountain (Westfjords, before and after ice loss. These observations allow us to propose a Fig. 1b). The of both areas falls within the Tertiary Basalt definition of the term to adopt for molards in permafrost terrains: Formation (Miocene-Lower Pliocene) (Jóhannesson, 2014), and is cones of loose debris that result from thawing of frozen blocks composed of basalts, rhyolites and weathered tephra layers. Both of ice-rich sediments mobilised by a landslide in permafrost ter- landslides originated from ice-cemented talus deposits perched . Our data dispel any lingering doubts about the direct link on topographic benches. The Móafellshyrna landslide occurred on between these landforms and permafrost degradation in cold en- 20th September 2012 at 873 m a.s.l., travelled ∼1,300 m and mo- vironments. We show that molards in permafrost environments bilised ∼310,000 m3 of material, while the Árnesfjall landslide are a critical indicator of permafrost degradation, and they can occurred on 14th July 2014 at 418 m a.s.l., travelled ∼560 m and reveal important information on the mobility of landslides in ter- had a volume of ∼130,000 m3. The preparatory factor for both rains affected by permafrost. We finally detail characteristics that landslides was intense , whilst permafrost degradation can be used to identify them in the field and from remote sensing was the main triggering factor, with a minor role of seismic activ- data. 138 C. Morino et al. / Earth and Planetary Science Letters 516 (2019) 136–147

2. Methods the GCPs in the image set, importing their coordinates recorded in the field by dGPS. The model was georeferenced using the 19 GCPs, 2.1. Fieldwork for which we obtained a horizontal uncertainty of 0.05 m to 0.08 m, and a vertical uncertainty of 0.05 to 0.15 m. The 3D model has We conducted fieldwork of the Móafellshyrna landslide nine reconstruction errors of 0.9–1.8 pixels and an absolute precision of days after its occurrence and of the Árnesfjall landslide one day af- 0.5–1.5 m. We then imported the DEM and the orthomosaic into ter the event. We revisited the Móafellshyrna site in summer 2015, ArcGIS for further analysis. three years after the landslide event, to perform field observations and ground-support for the collection of airphotos and airborne 2.4. Morphometric analysis of molards LiDAR (Light Detection And Ranging) data. We revisited the Árnes- fjall site in summer 2016 to perform field observations and collect In order to link the landslides’ processes to the morphology of a Structure from Motion (SfM) photogrammetry dataset. We also the molards, we i) analysed the molards’ distribution and geomor- performed differential GPS (dGPS) surveys of the landslides us- phic characteristics, ii) compared the slope angle of their longer ing two GNSS Leica System 1200 units in the Móafellshyrna site and shorter flanks, iii) calculated their eccentricity, and iv) plot- and two GNSS Leica VIVA GS10 System units in Árnesfjall site. One ted the orientation of their long axes with respect to those of the was used as a rover unit and the other one as base station, with landslides. the mean accuracy of measurements being 1 cm in the horizontal and 2 cm in the vertical direction. We investigated the interior of 2.4.1. Area, height, volume and slope characteristics one molard at each site to determine their inner composition and The molards were first digitised as a polygon in ArcGIS using structure. their contrast in terms of texture and shading in the orthomosaics. The area of molards was estimated by calculating the area of the 2.2. Airborne data resulting polygon (Fig. 6e). To measure their height (Fig. 6e), for each molard we used a topographic profile placed parallel to the In September 2015, the U.K. Natural Environment Research contour lines outside that molard and passing through its peak; Council’s Airborne Research Facility (NERC-ARF) on behalf of the the peak’s elevation was read directly from the profile, with a European Facility for Airborne Research (EUFAR) collected aerial representative elevation immediately outside the molard taken as photography and LiDAR data for the Móafellshyrna area in Iceland, its base. The elevation data for these profiles were derived from three years after the Móafellshyrna landslide occurred. 170 aerial LiDAR in the case of Móafellshyrna and SfM in the case of Árnesf- photographs were collected with a Leica RCD105 digital camera, jall. The volume of material composing the molards was calculated and 15 lines were flown to collect 126 million LiDAR points with following Conway and Balme (2014), reconstructing the surface 1.7 points/m2 using a Leica ALS50-II. A GNSS Leica System 1200 of the landslide without the molards, and deriving the deposited dGPS was used in a fixed location at 1 Hz during the flight to volumes by subtracting this surface from the surface with the mo- collect base station data for the on-board dGPS. The geometric lards (Fig. 6e). Error propagation calculations by Conway and Balme correction of the LiDAR point cloud was performed by NERC-ARF- (2014)suggest that such volume estimates are accurate to within DAN (Data Analysis Node) using the dGPS and navigational data. 15%. Slope of the basal area where molards lie was calculated by We used the LAStools extension for ArcGIS to convert the point creating a slope map of the surface of the landslide without the clouds into gridded Digital Elevation Model (DEM) at 1 m/pixel, molards and taking the mean value. The slope angle for the long using the return time of the last peak of light to reach the re- and short flanks of molards was the mean value of the slope ob- ceiver from the LiDAR laser shot (e.g., Höfle and Rutzinger, 2011; tained from the slope map of the original DEMs (Fig. 6e). Jaboyedoff et al., 2012;Roering et al., 2013), which is usually as- We performed two-sample t-tests assuming unequal variances sumed to be the ground return. We used Agisoft Photoscan Profes- for the volume, area, height, slope angle of the basal area and sional 1.3.5 to produce a seamless orthomosaic from the airphotos, eccentricity of molards, using the values of Móafellshyrna and Ár- where the position of the images was controlled using ten well- nesfjall respectively as variables. We used the values of each site spread ground control points (GCPs) derived by locating matching separately for testing the slope angles of short and long flanks of positions between a hillshaded version of the LiDAR DEM and the their molards (see Supplementary Table 1). airphotos. The combined uncertainty from the GCP placement and A dispersion parameter ψ = σ /μ (where σ is the standard the processing with Agisoft is ∼1m horizontally. deviation and μ is the mean) for height, axes and volumes of mo- lards, show less homogeneous sizes in Móafellshyrna compared to 2.3. Structure from motion those of Árnesfjall (see Supplementary Table 2). This is confirmed by Pearson correlation coefficient calculated for height-long axis, For the Árnesfjall site, through the Structure from Motion (SfM) height-short axis and long-short axis, showing a strong correlation photogrammetry technique (Carrivick et al., 2016;Smith et al., in Móafellshyrna, suggesting a circular shape in plan-view and ca- 2015;Westoby et al., 2012)we produced a 3D topographic model, sually linked dimensions, while in Árnesfjall lower values suggest from which we also derived an orthomosaic at 9 cm/pixel and DEM more elliptical shapes (see Supplementary Table 2). at 18 cm/pixel. Photos were taken using a hand-held single-lens The volume-frequency distribution for the molards in Móafell- reflex (SLR) camera (Canon EOS 450D, 12.2-megapixel image sen- shyrna shows a power-law distribution, while the volume-frequency sor) from a ground-based oblique perspective at approximately 3 distribution for the molards in Árnesfjall conforms to an exponen- km from the landslide, using a fixed focal length of 200 mm with tial distribution (see Supplementary Fig. S1). Both were determined automatic exposure settings enabled. We identified clearly visible by comparing three different plots of the volume-frequency distri- blocks and other features on the landslide — a total of 19 GCPs bution (i.e., linear-linear, logarithmic-linear, bi-logarithmic), where — and obtained their coordinates by using dGPS measurements. we considered the best distribution the one fitting the more to a Prior to processing, photographs were inspected manually, blurry straight line. images were deleted, and the sky was masked out of each im- age manually. The remaining 73 photographs were imported into 2.4.2. Eccentricity and direction Agisoft Photoscan Professional 1.4.1. We removed any misaligned To measure the eccentricity of the molards, an ellipse was fit- photographs after the initial alignment stage and then identified ted to approximate the perimeter of each molard, with the area of C. Morino et al. / Earth and Planetary Science Letters 516 (2019) 136–147 139

Fig. 2. The formation of molards. In each pair of images, ice-cemented blocks are on the left; the resulting molards are on the right. a, Cubic and angular blocks of ice-rich sediments (blue arrows) at the Móafellshyrna site that are perched on the source area and have fallen onto the talus slope below (the block in the foreground is ∼7 m wide). Photo taken the day after the failure (courtesy of G. Hansson). b, Similar perspective as in panel a, but three years after the failure, with remnants of the blocks of ice-rich sediments preserved as molards (blue arrows; the molard in the foreground is ∼9 m wide). c, Angular ridges generated by tilting of ice-cemented talus at the Árnesfjall site (up to ∼4 m high). Photo taken two days after the failure (courtesy of V. Benediktsson). d, Similar perspective as panel c, but two years after the failure, where remnants of the ice-cemented ridges have degraded into densely-packed, elongated mounds of debris (up to 3.7 m high). e, The largest block of ice-rich sediments that fell during the Móafellshyrna event; note different layers of deposits composed of imbricated boulders, cobbles, and pebbles embedded in brown to red sandy to silty (the white bar next to the person as a scale indicates 2 m height). f, The remnant of the block in panel e three years after the failure, preserved as a molard (the white bar next to the person as a scale indicates 2 m height). g, Side-view schematic evolution of a molard, from the block of clast-supported imbricated talus deposits cemented by ground-ice (bluish colour) to the collapsed conical mound of debris, preserving sorted deposits on its surface (dark brown), with fine deposits (light brown) being leached out. (For interpretation of the colours in the figure(s), the reader is referred to the web version of this article.) 140 C. Morino et al. / Earth and Planetary Science Letters 516 (2019) 136–147

Fig. 3. Maps of the landslides. a. A contour map with a line map of the Móafellshyrna landslide, with its main features and perimeter and location of the molards indicated. b. A contour map with a line map of the Árnesfjall landslide, with its main features and perimeter and location of the molards indicated. each ellipse being equal to the area of the perimeter of the repre- c) both during and immediately after the failure. We examined sented molard. Then, the eigenvalue and eigenvectors of each zone the lithological and grain-size composition of the blocks and ridges were calculated, with the orientation of the ellipse being in the of ice-rich sediments. They comprised slightly imbricated angular direction of the first eigenvector (Jorge and Brennand, 2017). Fi- boulders, cobbles, and pebbles embedded in a variably abundant nally, the geometric characteristics of the ellipse were calculated, sandy to silty clay matrix. The colour of the matrix varied from specifically the centroids, the major and the minor axes (the ratio brown to red, and the lithology of the clasts reflects the bedrock of the major and minor axes of the ellipse is the same as the ratio of the area (Tertiary Basalt Formation), with abundant fines origi- of their eigenvalues). We plotted the orientation of the long axes nating from the weathered volcanic lithologies. of the molards against the propagation direction of the landslides We revisited the Móafellshyrna and Árnesfjall sites 2 and 3yrs using GeoRose open-source software (Fig. 6b, d). after the failures, respectively. In Móafellshyrna, we found conical molards with rounded summits in the place of the original ice- 3. Results cemented blocks (Fig. 2b, f). In Árnesfjall, the elongate ridges in a group below the main scarp had become smaller (with maxi- 3.1. Molard formation in permafrost terrains mum height of 3.7 m) molards with the same elongated shape and pointed summits (Fig. 2d). Therefore, at both sites the molards are During the field observations performed immediately after the secondary features generated by the thaw of the ice cementing the failures, in the Móafellshyrna landslide deposits we found isolated frozen blocks or ridges, which were produced as primary features pseudo-cubic and angular blocks of ice-rich sediments (Fig. 2a, e) by the two landslides. At both sites, at the surface most molards that came to rest at the foot of the talus slope where the topogra- are clast-supported (by boulders, cobbles and pebbles; Figs. 1c, 4a), phy flattens (Fig. 2a), 295-390 m below the source area (Fig. 3a). At with fine material sometimes present, but there are rare cases of Árnesfjall, we found a spatially dense group of angular, elongated molards composed of matrix-supported gravelly to silty clay ice-cemented ridges of debris 40-150 m below the top of the main material (Fig. 1d). However, the surface material does not nec- scarp of the landslide (Figs. 2c, 3b), and an isolated block of ice- essarily reflect the inner composition of molards; in the interior, rich sediments at the toe of the landslide. In both landslides, the the deposits are richer in matrix just a few decimetres below the blocks and ridges were composed of poorly-sorted clast-supported surface (Fig. 4b). Once the blocks disaggregated into molards, we talus deposits. Ground-ice was cementing the material, allowing observed that some of them still preserved a faint hint of the the blocks and ridges to preserve a cubic or angular shape (Fig. 2a, original layering of the source deposits (Fig. 1c), with imbricated C. Morino et al. / Earth and Planetary Science Letters 516 (2019) 136–147 141

ice-cored moraines are similar to molards not only in their conical shape, but also in their mode of formation: ice-cored moraines re- tain massive ice cores and form hummocks when the ice melts (Dyke and Savelle, 2000). Similarly to molards, the form of the hummocks is determined by thaw and gravitational collapse. They ◦ can have similar flank slope angles to molards of between 25 and ◦ 35 (Hambrey et al., 1997)or higher. Many meters of ice can be buried under only a few centimetres to meters of till (Schomacker and Kjær, 2008), meaning that ice-cored hummock can easily dis- Fig. 4. Material comprising molards. a, A molard in Móafellshyrna showing coarse appear completely when all the ice is melted (Mercer, 1963). The deposits on the surface with no gradation or preferential orientation (measuring ratio of ice to debris within molards in Móafellshyrna is much tape extended to 1 m as scale). b, Internal composition of a molard, comprising lower (15-20% ground ice content estimated from visual inspec- unsorted gravel in sandy silt to silty clay matrix (the pen is 10 cm long). tion; Sæmundsson et al., 2018), giving them a much higher preser- vation potential than hummocks in ice-cored moraines. This is why unsorted gravel in sandy silt to silty clay matrix, which is further molards remain recognisable even after the ice has gone. proof that the deposits must have been ice-cemented during the We posit that molards should vary in shape and size depend- failure. However, in most of the cases, molards comprised non- ing on i) the slope on which they form, ii) their ice content and stratified unsorted gravel to clay material, and we did not observe granulometry, and iii) the emplacement dynamics of the host land- any radial grain size gradation (Fig. 4a, b). slide. Circular molards with rounded summits are found on rel- ◦ A schematic evolution of the degradation of ice-cemented de- atively low slopes (<17 in Móafellshyrna), whereas those with bris material into a molard has been recently proposed (Milana, angular summits and elongate shapes are found on relatively high ◦ 2016), where the author assumed a sphere as the theoretical initial slopes (28 in Árnesfjall). Based on arguments of shape of the frozen mass. We found that molards with different for loose granular materials (Carrigy, 1970;Pohlman et al., 2006; shapes are a direct result of the different shapes of the original Van Burkalow, 1945), molard shape should vary with granulom- blocks of ice-cemented material. From a cubic or angular block, etry, but we have no specific observations to substantiate these the resulting molard has a rounded and symmetrical conical shape relationships. Our observations show that molards in permafrost (Fig. 2e-g); alternatively, in the case of ice-cemented elongated terrains derive from the thaw of the ground ice cementing loose deposits, therefore molards cannot form without ground ice. This ridges, the final molards preserve the elongated shape. In both cements the loose material, allowing it to behave like a solid dur- cases, we found that molards can have up to 30% lower relief than ing transport, and once thawed induces collapse of the material the initial block, because thaw results in the debris located at the into molards. Our observations suggest that the higher the initial top of the block/ridge collapsing to its base (Fig. 2g). Assuming that ground ice content the lower the final height of the molard, com- ice only fills the pore-space, we can estimate the original size of pared to the initial frozen block, hence, the lower its flank slopes a block of ice-rich sediments from the resulting molard. Knowing and the more domed its summit. If the amount of ground ice is the height (h) and length of the flank (L) of a molard, it is possi- far in excess of the pore space, we expect them to behave like ble to estimate the width (B) – and then the area and volume – of hummocks in ice-cored moraines. We infer that the lack of matrix the original block as shown in equation (1)(Fig.5): observed at the surface of the molards years after their deposition  and degradation is due to the action of precipitation, washing out B = 2 L2 − h2 (1) the fines through the coarse debris by percolation (Fig. 2g). Finally, the shape and size of the molards also depends on the mechanism We have tested this model through field measurements of the of emplacement of the landslide, and we report on this in the fol- size of the largest block of ice-rich . A molard of L = 11.9 lowing section. m and h = 10.2m will result from a block with B = 12.2m, which is approximately the measured length of the biggest block 3.2. Molard distribution and morphology: indicators of landslide in Móafellshyrna after the failure. So for example, from a cubic and dynamics angular block 10.2 to 14.8 m high and with an area from 924.24 to 1002 m2, the resulting molard would have a rounded and symmet- The morphometry and the spatial distribution of molards in rical conical shape, would stand 10.2 m high maximum and cover Móafellshyrna and Árnesfjall landslides provide information about an area of 1193 m2. If molards obey this relation, then this model the failure dynamics of the landslides. Molards act like tracers, be- gives an estimation of the maximum initial ground ice content of cause during transport they behave like solid blocks and thus we the original blocks. know where they originate. Molards in Móafellshyrna have dif- ferent characteristics to those in Árnesfjall, although the average height, basal area and volume of the molards do not differ signif- icantly between the two landslides (Supplementary Table 1). For instance, the Móafellshyrna molards have a larger range in vol- umes (Fig. 6e). Also, the slope angles of the short and long flanks of molards are roughly equal in Móafellshyrna (Fig. 6e), while the downslope faces of the Árnesfjall molards are longer and less steep than the scarp-facing short flanks (Supplementary Table 2, Fig. 6e). The Árnesfjall molards are consistently elliptical in shape (Figs. 3b, 6e), whereas molards in Móafellshyrna are variable, but generally Fig. 5. From block to molard. Model for estimating the width of the original block of ice-rich sediment (assumed to have a rectangular section) knowing height and closer to circular (Figs. 3a, 6e). Finally, while Móafellshyrna mo- length of the flank of the molard, which is assumed to have a triangular section. lards do not show a preferential orientation in their long axes with respect to the landslide’s runout-direction (Fig. 6b), the long A similar surface lowering (35%) has been recorded in hum- axes of the molards in Árnesfjall are oriented roughly perpendic- mocks of thawing ice-cored moraines in the terminal region of ular to the landslide’s runout direction, and parallel to the main outlet- in Iceland (Krüger and Kjær, 2000). Hummocks of scarp (Fig. 6d). 142 C. Morino et al. / Earth and Planetary Science Letters 516 (2019) 136–147

Fig. 6. Dimensions and distribution of molards. a, The topographic profile of the Móafellshyrna landslide, with the position of molards marked as grey points, and the main scarp zone marked with a thick grey line. b, Rose diagram of the orientation of molards’ long axes where the orientation of the Móafellshyrna landslide is marked as a hatched texture and by an arrow. c, The topographic profile of the Árnesfjall landslide, with the position of molards marked as grey triangles, and the main scarp zone marked with a thick grey line. d, Rose diagram of the orientation of molards’ long axes where the orientation of the Árnesfjall landslide is marked as a hatched texture and by an arrow. e, Boxplots of the volume, area, height, slope of the basal area, slope of the short and long flanks, and eccentricity of the molards in Móafellshyrna and Árnesfjall; note that the vertical axis for volume and area use a logarithmic scale. The whiskers indicate the maximum and minimum values, the boxes mark the interquartile range, and the black lines are the medians of each population. For all the measurements, in Móafellshyrna n = 17, in Árnesfjall n = 23.

In Móafellshyrna, the molards are clustered in an area with a facing short flanks of these molards (Figs. 2c, d, 6e) reflects a ◦ mean slope of 17 at the foot of, or beyond, the talus slope located common behaviour of rotational slides, where the upper part of below the topographic bench (Fig. 6a). Their position matches that the units produced by curved rupture surfaces tilt backwards to- expected for , where competent fragments detach and fall, ward the scarp. and come to rest either within, at the foot, or beyond the base of By measuring the size, morphology and distribution of molards, the talus slope (Evans and Hungr, 1993; Varnes, 1978). The large we have been able to discriminate between the processes of sim- size variation of molards in Móafellshyrna mirrors that in - ple gravitational fall in the Móafellshyrna case and of rotational fall processes (Fig. 6e), where their power-law frequency–volume sliding in the Árnesfjall case. We recognise that other types of mo- distribution (Supplementary Fig. S1) is related to rock fragmen- tion might also transport ice-cemented materials, forming molards, tation (e.g., Dussage-Peisser et al., 2002; Dussauge et al., 2003; once the ice has thawed, with morphometry and distribution dif- Einstein, 1937). Their final position on a low slope allowed the ice- ferent to those described here. For example, molards with a wide- cemented blocks to degrade into isolated molards with a circular range of dimensions scattered across the mobilised mass could be base, radially symmetrical flank slopes (see Fig. 6e, Supplementary expected in rock/debris avalanche deposits, due to the entrainment Table 2), and no preferential orientation. of the ice-cemented blocks in their extremely rapid flow-like mo- ◦ Molards in Árnesfjall lie on an average slope of 26 and are tion (Hungr et al., 2001). Molards accumulated in the terminal lobe concentrated less than 30 m below the base of the failure scarp of debris-flow deposits could result from the degradation of blocks (Fig. 6c). This position is typical of the location of en echelon of ice-rich sediments transported by the flow, due to their ten- concave-upward rupture surfaces in debris slides (Hungr et al., dency of producing longitudinal sorting near the front of the surge 2001). These ruptures are oriented perpendicular to the flow di- (Iverson, 1997). rection, and blocks of material often move downward with little internal deformation (Varnes, 1978). Molards in Árnesfjall derive 3.3. Molards in remote sensing data from the degradation of densely-grouped, elongated ice-cemented ridges exposed by the rotational-sliding motion of the source ma- Identifying molards reliably is crucial if they are to give insights terial, by several curved planes of movement. These dynamics into the geomorphological and climatic condition of the landscape are reflected in their more homogeneous size (see Supplementary where they form. Table 1) and their elliptical shape (Fig. 6e), a result of the degrada- In addition to molards being readily identifiable in the field, we tion of elongated cusps of ice-cemented precursor units produced propose that in most cases they can also be reliably identified in by relatively coherent rotational sliding. This process can be de- remote sensing data, although field studies would be needed to duced by the orientation of the molards’ long axes, perpendicular confirm their identification as molards. In Fig. 7, we show a plan to the main movement downslope (Fig. 6d). The steep upslope- view image of molards in Móafellshyrna landslide (Fig. 7a, b), a lo- C. Morino et al. / Earth and Planetary Science Letters 516 (2019) 136–147 143

Fig. 7. Molards from remote sensing. a, Aerial image of Móafellshyrna landslide, with the white box outlining the extent of panel b. b, Molards in Móafellshyrna landslide. White arrow in panel a indicates the lighting direction in panels a and b. c, Aerial image of a rock avalanche on the south coast of Nuussuaq, west coast of (image from Google Earth) with the white box outlining the extent of panel d. d, Candidate molards in Nuussuaq (image from Google Earth). White arrow in panel c indicates the lighting direction in panels c and d. e, Impact ejecta flow deposits from Hale Crater on Mars, (Context Camera images G14_023858_1429 and G19_025704_1431, credit NASA/JPL-Caltech/MSSS), with black arrows indicating the inferred flow directions and white box indicating the position of panel f. f, Conical landforms in the ejecta blanket of Hale Crater on Mars (High-resolution Imaging Science Experiment image ESP_052222_1425, credit NASA/JPL/University of Arizona). White arrow in panel e indicates the lighting direction in panels e and f. cation with candidate molards elsewhere on Earth (Fig. 7c, d), and 2002)in Greenland. The failure falls within a region where land- possible candidates on Mars (Fig. 7e, f). In Fig. 7d, cone-shaped slides have been active for the last 10,000 yrs (Dahl-Jensen et al., landforms of ∼1-46 meters-diameter are scattered across the sur- 2004) and continuous permafrost has been reported (Van Taten- face of deposits mobilised by a rock avalanche (Pedersen et al., hove and Olesen, 1994). Hence, we infer that these conical mounds 144 C. Morino et al. / Earth and Planetary Science Letters 516 (2019) 136–147 are molards, and in situ investigations have confirmed this hypoth- ( 2016)moved from a morphological to a process-related defini- esis (Benjamin et al., 2018;Pedersen et al., 2001; Stuart Dunning, tion of molards, as they hypothesised that landslides in permafrost pers. comm.) providing strong evidence of the occurrence of per- terrains could mobilise blocks of frozen material, which degraded mafrost degradation in this area. Many more candidate molards into cones of debris – molards – once at rest. Since in the past are seen here than in our Icelandic examples, suggesting that more the terms “molards” and “hummocks” have been used interchange- molards could be produced in areas with more pervasive ground ably, which can lead to confusion, and since the majority of the ice. cases of molards in the literature are reported to have formed in Similar scattered conical landforms are identifiable within im- permafrost/cold terrains (Brideau et al., 2009;Jermyn and Geert- pact ejecta originating from Hale Crater on Mars (Fig. 7e, f). The sema, 2015;Lyle et al., 2004; Milana, 2016;Schwab et al., 2003; ice-rich nature of the impact ejecta has already been proposed Xu et al., 2012), a refinement of the definition of molards occur- based on other landforms (e.g., Jones et al., 2011). We propose that ring in permafrost environments is necessary. Here, we develop on conical landforms within the ejecta flow are molards, which with the recent definition of molards of Brideau et al. (2009) and Milana further study could yield important constraints on the ice-content (2016)by providing the missing field evidence to demonstrate the of the original ejecta flow and insights into recent modification of entire formation-history of molards in permafrost environments ground ice reservoirs on Mars. that allow us to refine their definition: cones of loose debris that derive from the thaw of blocks of ice-rich sediments mobilised 4. Discussion by landslides in permafrost terrains. They are secondary features, resulting from the primary emplacement of initially frozen ice- Iceland has discontinuous permafrost (Rekacewicz, 2005), which cemented blocks/ridges within landslides, which degrade into con- in general is spatially and temporally heterogeneous (Rödder and ical mounds as the ice thaws in their new position. Molards re- Kneisel, 2012), and mountain permafrost has been modelled at ported in landslides have often involved terrains that are known elevations above 800-900 m a.s.l. in the central-northern regions to be affected by discontinuous permafrost (Brideau et al., 2009; of the island (Etzelmüller et al., 2007). This predicted permafrost Lyle et al., 2004). In one case, the deposits of an active rock distribution has been ground-truthed by comparison with the dis- were the source material for the landslide with molards tribution of active rock glaciers and ice–cored moraines in the (Milana, 2016). As we have shown that molards in cold environ- Tröllaskagi peninsula (Farbrot et al., 2007; Lilleøren et al., 2013). ments evolve from parent blocks of ice-rich sediments, insights The Móafellshyrna landslide falls within the predicted spatial zone from this study allow previously reported molards in permafrost and altitude band for mountain permafrost. In contrast, the Árnes- environments to provide new information: they can be an indi- fjall landslide occurred on the coastline of the Westfjords, 400 m cator (perhaps the only indicator) of ongoing or past permafrost below the predicted permafrost altitude. However, at the Árnesf- degradation in permafrost terrains. In Móafellshyrna, molards are jall site the elevation for permafrost could be significantly lower the first direct indication of ongoing permafrost degradation in than the one predicted by Etzelmüller et al. (2007) and Farbrot et this region of Iceland, in an area already thought to host moun- al. (2007), because of lower summer temperatures, shorter melt- tain permafrost (Etzelmüller et al., 2007;Farbrot et al., 2007; ing season and the proximity at less than 25 km of the Dran- Lilleøren et al., 2013). In Árnesfjall, molards mark both the pres- gajökull ice cap, around which the presence of permafrost has ence of permafrost in an area thought to have no perennially been hypothesised, but never confirmed (Brynjólfsson et al., 2014; frozen ground and its recent/ongoing degradation. Hjort et al., 1985). Hence, the presence of molards here is the Our new recognition that molards provide a way to per- only unequivocal and pragmatically detectable indicator of per- mafrost degradation is important, as similar indicator-landforms mafrost conditions. Both landslides originated from talus material. (such as retrogressive-thaw slumps (Ashastina et al., 2017), ther- In general, coarse blocky talus material is a particularly favourable mokarst lakes (Yoshikawa and Hinzman, 2003), baydjarakhs substrate for ice-rich permafrost (Etzelmüller et al., 2001;Harris (Séjourné et al., 2015)are scarce, and generally occur only in and Pedersen, 1998). In talus slopes, ground ice can be formed zones of continuous permafrost. Other indicators of permafrost, by burial of snow by debris, which protects the such as active rock glaciers, ice-cored moraines, composite ridges, snow from ablation (Gruber and Hoelzle, 2008), and can lead to ice-wedges, or , need long-term monitoring of air/ground the development of ice-rich permafrost (Kenner et al., 2017). An- temperature to detect the state of permafrost, whereas molards nual freeze-thaw cycles in talus slopes in mountain permafrost directly reveal permafrost degradation. We propose that future environments can reach depths of ∼3-7 m, but rarely decametres studies can use molards with the purpose of mapping historic- (Matsuoka et al., 1998). The ice-cemented blocks that detached in permafrost conditions and geographically tracking past climate the Móafellshyrna landslide were 15-20 m thick, much larger than change. the expected maximum depths of annually-formed, ice-cemented Molards could perhaps be confused with hummocky terrains ground. Hence, the source talus slopes were perennially-frozen found in debris/rock-avalanche deposits. Hummocks are mounds, ground and the molards here indicate permafrost degradation. Fur- blocks and ridges that characterize large landslides and debris/rock thermore, permafrost degradation due to anomalous rise in tem- avalanches (Paguican et al., 2014;Shea and van Wyk de Vries, perature has been recognised as the main trigger for the Móafell- 2008;Siebert, 1984;Ui, 1983;Yoshida et al., 2012). Hummocks shyrna landslide (Sæmundsson et al., 2018). result from the formation of horsts and grabens during transport In the few works where they have been reported, molards are and spreading or by stretching around blocks by faults (Davies et generally defined as conical mounds occurring in landslide de- al., 2013;Glicken, 1996;Glicken et al., 1981;Shea et al., 2008; posits (Cassie et al., 1988;Goguel and Pachoud, 1972; Mollard Voight et al., 1981), and their shapes can be used to determine and Janes, 1984). However, the application of the term “molard” the kinematics of a landslide (Shea and van Wyk de Vries, 2008). has not been consistent. For example, Cassie et al. (1988)de- Hummocks in debris/rock-avalanche deposits are characterised by scribed “molards” in the Frank Slide, following the terminology of pervasive shattering or jigsaw fracturing as result of brittle defor- Mollard and Janes (1984): “one of a series of conical mounds of mation associated with the transport of large rock masses (e.g., broken slide rock deposited along the typically lobate margin of Calvari et al., 1998; Shreve, 1968; Ui et al., 1986; Yarnold and rock avalanche spoil debris; a debris cone”. However, in later liter- Lombard, 1989). Hence, hummocks should differ sedimentologi- ature (e.g., Charrière et al., 2016), the same mounds at Frank Slide cally from molards in permafrost terrains, but this would require are called “hummocks”. Recently, Brideau et al. (2009) and Milana further data collection to fully substantiate. Many processes can C. Morino et al. / Earth and Planetary Science Letters 516 (2019) 136–147 145 generate conical mounds, but molards in permafrost terrains are disastrous ground ice thaw-induced landslides in vulnerable areas. always associated with a landslide. Molards in permafrost terrains We have demonstrated that molards are a landform that can be would not form without ground ice; this makes the debris mate- readily recognised in the field as a marker of recent and ongo- rial behave like solid blocks during the failure and transport be- ing permafrost degradation. We found relatively few molards in cause of its cementing action, and allows the formation of discrete Iceland compared to in previous studies of molards in permafrost- mounds of loose debris (molards) via melting. No ice is involved rich terrains, hence the number-density of molards at a site may or is necessary in the formation of hummocks, while it is instead reveal the abundance of ground ice at the time of the failure. the primary factor in the formation of molards in permafrost en- One of our study sites in northern Iceland falls in an area where vironments. When studying molards, it is important to consider mountain permafrost was not predicted (Etzelmüller et al., 2007; the geomorphological and environmental setting where they are Rekacewicz, 2005), and until our study there was scarce direct field found. There are examples in the literature of other mounds and evidence of permafrost and/or its condition. The recent formation ridges that have similar characteristics to the molards reported of molards reveals the presence of permafrost in the area, and its here. Some have been observed in mountainous non-permafrost ongoing degradation. terrains, others at the limits of discontinuous permafrost, or in Finally, we have demonstrated that molards are not only read- paraglacial environments, but have not been linked to permafrost ily recognisable in the field, but also via remote sensing. This opens degradation (Blais-Stevens et al., 2015;Dufresne et al., 2018; up the possibility of efficiently identifying these landforms across Geertsema et al., 2006a;Schwab et al., 2003). These include for large and remote areas, revealing the influence of ice thaw not example ridges and hummocky that are considered ev- only in mountain permafrost environments, but also on plane- idence of spreading during the motion of the Mink Creek landslide, tary surfaces such as Mars, where the role of volatiles in land- north-western British Columbia, involving glaciomarine sediments scape evolution is vigorously debated (Conway and Balme, 2016; in a non-permafrost area (Geertsema et al., 2006a), and simi- Dundas et al., 2017; Leverington, 2011). lar ridges were observed in other spreading processes in Québec (Carson, 1979). Conical features and ridges up to 5 m high are thought to have been produced by the fragmentation of basalt- Author contributions boulders transported by the Sutherland landslide, British Columbia, in an area not affected by permafrost (Blais-Stevens et al., 2015). C.M., S.J.C., Þ.S., J.K.H., and M.R.B. conceived and designed the In 2015, a landslide occurred at the terminus of Tyndall Glacier research. C.M. collected, processed, and analysed the data, and pre- in , traversed the width of the Taan fjord, and re-emerged pared the manuscript. S.J.C. significantly contributed to data col- onto land depositing mounds and hummocks that preserved the lection and analysis and manuscript preparation. Þ.S. and J.K.H. source stratigraphy, but no ice was observed within the deposits contributed to data collection and manuscript preparation. J.H. (Dufresne et al., 2018). Molards have been reported in a recent contributed to data analysis and manuscript preparation. F.E.G.B. rock avalanche in west central British Columbia, where the role of collected data for Mars and contributed to manuscript prepara- degrading mountain permafrost in the release of the failure is in- tion. M.R.B. and C.J. contributed to data processing and manuscript ferred, but the permafrost parameters were not studied (Schwab et preparation. T.A. contributed to manuscript preparation. al., 2003). In the light of our study, the features listed above might Competing interests: The authors declare no competing inter- be seen with new eyes and reinterpreted as molards, or confirmed ests. as hummocks on collecting new data. Further studies should there- fore focus on distinguishing molards from other conical features on Acknowledgements landslides in order to avoid confusion in their interpretation.

5. Conclusions This work has been funded by a postgraduate studentship grant (NE/L002493/1) from the Central England Natural Environment Re- By studying how the deposits of two landslides in northern Ice- search Council Training Alliance (CENTA). This project forms part land evolved through time, we have shown for the first time that of and is funded by a British Geological Survey BUFI CASE Stu- molards in permafrost terrains are cones of loose debris that re- dentship (GA/14S/024, Ref: 284). We thank the Natural Environ- sult from the degradation of blocks and ridges of ice-cemented ment Research Council Airborne Research Facility (NERC ARF) and deposits mobilised by landslides. Molards have distinctive spatial their Data Analysis Node (NERC-ARF-DAN) plus the EUropean Fa- and geomorphic characteristics that reveal the dynamics of the cility for Airborne Research (EUFAR) for the air photography and landslides that formed them. Isolated molards like those of the LiDAR data on which this paper relies. Thanks go to NERC Geo- Móafellshyrna landslide indicate transport via fall, while densely- physical Equipment Facility (GEF) for the Loans 1048 and 1064 grouped elongated molards below the main scarp like those of through which differential GPS surveys were possible. We thank the Árnesfjall landslide are generated by sliding. We distinguish the GeoPlanet writing residential scheme for the support in writ- two different dynamic styles using molards, but future study is ing this manuscript. C. Jordan publishes with permission of the required to determine if molard characteristics could reveal other Executive Director of BGS. S. J. Conway is funded by the French types of landslide motion. The relation between permafrost degra- Space Agency, CNES, for her HiRISE related work. We gratefully dation and slope stability is well documented in the literature (e.g., acknowledge the contribution of Gestur Hansson (Icelandic Meteo- Beniston et al., 2018; Haeberli et al., 2010;Huggel et al., 2012; rological Office) and of our field assistants Francesco Giuntoli, Silvia Kellerer-Pirklbauer et al., 2012; Phillips et al., 2017), but, in the Crosetto, and Sydney Gunnarson. The authors thank the reviewers light of our study, further efforts should be engaged in defining Anja Dufresne, Marten Geertsema and Gioachino Roberti for their how molards can be used in hazard assessment. The types of land- insightful comments. slide from which molards form are likely to become more com- mon as ground temperatures in ice-rich permafrost zones increase, making thaw and slope instabilities in periglacial and glacial en- Appendix A. Supplementary material vironments more probable. Given that the presence of ice enables long runout and high velocity in mass movements (Huggel et al., Supplementary material related to this article can be found on- 2005), molards could be used to signal areas at risk of potentially line at https://doi .org /10 .1016 /j .epsl .2019 .03 .040. 146 C. Morino et al. / Earth and Planetary Science Letters 516 (2019) 136–147

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